73 
Physiology of the Genus Cusc7cta. 
epidermal cells exactly like those on other parts. The swelling 
produced by the growth of a constantly-developing haus- 
torium under a point of continued contact with a host, is 
broader and not so high, and is surrounded at a little distance 
by a ring-like elevation of about half its height. Longi- 
tudinal sections show, as I have already described and 
figured \ that the central elevation is formed by the pressure 
of the subjacent growing haustorium, and is covered by thin- 
walled, papillate epidermal cells of very considerable length 
and abundant protoplasmic contents ; that surrounding this 
are a few rows of shorter, but still papillate, epidermal cells of 
similar character ; that beyond these are several rows of long, 
papillate epidermal cells, these alone forming the elevated 
ring surrounding the central protuberance ; and that beyond 
this the epidermal cells remain typical in form and contents. 
We see plainly that not only is the formation of close spirals 
and of haustoria a result of contact-irritation, but that the 
changes in the epidermal cells are also due to this cause. As 
will presently be shown, however, the complete development 
of the haustoria and a complete modification of the overlying 
and immediately adjacent epidermal cells is not the result of 
contact only. 
Ludwig Koch 2 has clearly described that, when the parasite 
has made one or two close turns about a branch of its host 
and does not continue winding about this, but makes at once 
a similar spiral about the petiole of a leaf, which springs at 
that point from the stem of the host, that part of the Cuscuta 
between the last point of contact with the stem and the first 
point of contact with the leaf will, like the adjacent regions, 
bear haustoria. But the haustoria in this intermediate region 
will not develop far, and we shall see instead the conical 
swellings which Koch denotes ‘ sterile Haustorien.’ He ascribes 
their formation, like that of normal active haustoria, to irri- 
tation ; but he fails to make clear whether he believes them to 
be the result of irritation by contact at the points where they 
appear, or whether they are induced to form by the contact 
1 Loc. cit., p. 295. 2 Loc. cit., 1880, p. 50. 
